diff --git a/parser/outparam.py b/parser/outparam.py
index 76b1d81..9ae50d1 100644
--- a/parser/outparam.py
+++ b/parser/outparam.py
@@ -48,12 +48,27 @@ def doxygen_sources(meos_root: str | Path) -> list[str]:
 # here because, combined with this module's whole-doc-block `_FUNC` match, a DOTALL
 # `.*?` inside the skip run backtracks catastrophically (doxygroup runs a bounded
 # two-step search, so it can afford it); no vendored out-param carrier needs it.
+# A guard may also sit between the return-type line and the name, choosing between two
+# spellings of one signature (`basetype_in` in meos/src/temporal/type_in.c marks a
+# parameter UNUSED in the build that does not read it):
+#   bool
+#   #if CBUFFER || NPOINT || POSE
+#   basetype_in(const char *str, MeosType type, bool end, Datum *result)
+#   #else
+#   basetype_in(const char *str, MeosType type, bool end UNUSED, Datum *result)
+#   #endif
+#   {
+# `_TWIN` steps over the `#else`/`#elif` spellings and the closing `#endif`, so
+# `params` is the first spelling and the match still ends at the body's `{`.
 _SKIP = r"(?:[^\S\n]*(?:\#[^\n]*|//[^\n]*)?[^\S\n]*\n)*"
+_TWIN = (r"(?:[^\S\n]*\n[^\S\n]*\#[^\S\n]*(?:else|elif)[^\n]*\n[^;{]*?\))*"
+         r"(?:[^\S\n]*\n[^\S\n]*\#[^\S\n]*endif[^\n]*)?")
 _FUNC = re.compile(
     r'/\*\*(?P<doc>.*?)\*/[^\S\n]*\n' + _SKIP +
     r'(?:[A-Za-z_][\w\s\*]*?\n)?'          # optional return-type line
+    + _SKIP +
     r'(?P<name>[a-z][a-z0-9_]*)\s*\('
-    r'(?P<params>[^;{]*?)\)\s*\{',
+    r'(?P<params>[^;{]*?)\)' + _TWIN + r'\s*\{',
     re.S)
 # One @param[out] entry: capture the (possibly comma-separated) parameter names.
 _POUT = re.compile(r'@param\[out\]\s+(?P<names>\w+(?:\s*,\s*\w+)*)', re.S)
diff --git a/tests/test_outparam.py b/tests/test_outparam.py
index de2d6ff..feb4966 100644
--- a/tests/test_outparam.py
+++ b/tests/test_outparam.py
@@ -76,7 +76,81 @@ def _write(root, rel, text):
 """
 
 
+# A guard between the return-type line and the name choosing between two spellings
+# of one signature, the shape of `basetype_in` in meos/src/temporal/type_in.c, then
+# an undocumented neighbour that must not inherit the block.
+TWIN_SIGNATURE = """\
+/**
+ * @brief doc
+ * @param[in] str String
+ * @param[in] end True when the string must end where the value ends
+ * @param[out] result Value read
+ */
+bool
+#if CBUFFER || NPOINT || POSE
+basetype_in(const char *str, bool end, Datum *result)
+#else
+basetype_in(const char *str, bool end UNUSED, Datum *result)
+#endif
+{
+  return true;
+}
+
+/**
+ * @brief doc
+ */
+static Datum
+parse_coord(json_object *obj)
+{
+  return 0;
+}
+"""
+
+# A guard inside the parameter list, the shape of `skiplist_splice` in
+# meos/src/temporal/skiplist.c, then an undocumented neighbour.
+TWIN_TAIL = """\
+/**
+ * @brief doc
+ * @param[in] list Skiplist
+ * @param[in] sktype Type of the skiplist
+ */
+void
+skiplist_splice(SkipList *list,
+#if MEOS
+  SkipListType sktype)
+#else
+  SkipListType sktype UNUSED)
+#endif /* ! MEOS */
+{
+  return;
+}
+
+/**
+ * @brief doc
+ */
+void **
+skiplist_values(SkipList *list)
+{
+  return NULL;
+}
+"""
+
+
 class TestExtract(unittest.TestCase):
+    def test_guard_between_return_type_and_name(self):
+        with tempfile.TemporaryDirectory() as root:
+            _write(root, "meos/src/type_in.c", TWIN_SIGNATURE)
+            out = extract_outparams(Path(root) / "meos")
+            self.assertEqual(out.get("basetype_in"), ["result"])
+            self.assertNotIn("parse_coord", out)
+
+    def test_guard_inside_the_parameter_list(self):
+        with tempfile.TemporaryDirectory() as root:
+            _write(root, "meos/src/skiplist.c", TWIN_TAIL)
+            names = extract_param_names(Path(root) / "meos")
+            self.assertEqual(names.get("skiplist_splice"), {"list", "sktype"})
+            self.assertNotIn("skiplist_values", names)
+
     def test_plain_meos_src(self):
         with tempfile.TemporaryDirectory() as root:
             _write(root, "meos/src/temporal_tile_meos.c", PLAIN)